The field of the present invention relates to means and techniques for forming inclined boreholes. A particular application of the invention is the formation of inclined boreholes for placement of pilings or for directional wells. More specifically, the invention relates to drilling apparatus in which a drilling machine and pipe handling derrick or mast may be inclined to form an inclined borehole.
It is frequently necessary in the construction or placement of offshore drilling platforms that inclined pilings be placed into the water bottom as support for a drilling or production platform. These inclined pilings are frequently positioned with the use of a floating vessel which carries a pile driver employed for pounding the pilings into place. In deeper waters, pile drivers cannot be employed for this purpose, and in some circumstances, it is necessary to employ large, expensive drill ships for the purpose of setting the pilings.
Well drilling apparatuses in which the mast may be pivotal away from the vertical position are well known. These assemblies are commonly employed to drill directional bore holes in shallow water marine drilling operations. An example of such an assembly is shown in U.S. Pat. No. 3,443,647. In this assembly, a substantially conventional mast, traveling block and rotary table mounted on a pivotal substructure and equipped with means such that the angle of inclination of the mast, rotary table and substructure may be varied as required. The drill string is rotated by the rotary table and is raised and lowered with the traveling block, in a customary fashion.
While the patented assembly provides many advantages, it is nevertheless relatively expensive and difficult to mount a conventional mast and rotary table for pivotal movement and to operate such a system while it is in an inclined position. Even in the conventional vertical configuration, a relatively large support vessel is required to mount such a system. In a system where the mast is to be inclined, an even larger, more expensive and complicated support vessel may be needed. Operation of such vessels in many shallow water locations would not be possible.
A modified form of drilling equipment which provides both the rotary and vertical pipe movements required in a conventional drilling or completion operation is described in U.S. Pat. No. 3,722,603. This assembly, referred to herein as a drilling snubber, employs a drilling head and a gripping head which cooperate to raise and lower the well pipe and to impart the desired rotary motion to the pipe. As more fully described in said U.S. Pat. No. 3,722,603, the snubber includes a rotatable vertically movable upper drilling head and a stationary, rotatable lower gripping head. Hydraulic cylinders raise and lower the drilling head and additional hydraulic controls cause elements in both heads to grip and release the pipe as required. Both the lower and upper heads are equipped with means for imparting a rotary motion to the well pipe through the gripping elements of the head.
In the operation of the drilling snubber, rotary motion for drilling a well is imparted to the drill pipe through the drilling head. As the drill bit advances through the formation, the drill string is lowered by appropriately actuating the hydraulic cylinders which move the drilling head downwardly toward the stationary lower head. During the drilling operation, the lower head is completely released from the drill string. When the drilling head reaches the lower limit of its movement, the lower head is actuated to grip the drill string, the upper drilling head then releases the drill string and the hydraulic cylinders are actuated to elevate the drilling head. The drilling head then re-engages the drill string, the lower head releases the drill string and the drilling operation is reinitiated.
Deviation of the well direction away from the vertical is conventionally accomplished in a drilling snubber operation by using a whipstock or subsurface turbine drill, much as such devices are employed to deviate well bores being drilled with the use of a standard rotary table. These devices typically require that a vertical, or near vertical, guide hole be drilled before the deviation is effected. Shallow well and pile setting applications, however, require immediate deviation of the bore hole from the surface so that conventional deviating devices are of little use.